Type t = typeof(WindowsPrincipal);

                                    int rid = (int)WindowsBuiltInRole.PowerUser;

                                    object[] args = new object[1];

                                    args[0] = rid;

                                    string role = (string)t.InvokeMember("_GetRole",

                                                                        BindingFlags.Static |

                                                                        BindingFlags.NonPublic |

                                                                        BindingFlags.InvokeMethod,

                                                                        null,

                                                                        null,

                                                                        args);

Now [net fx 2.0] it is much easier and more important it is supported and one can use WellKnownSidType from System.Security.Principal namespace

http://msdn2.microsoft.com/en-US/library/system.security.principal.wellknownsidtype(VS.80).aspx

Also check on Keith Brown's relevant content:

http://www.pluralsight.com/wiki/default.aspx/Keith.GuideBook/HowToProgramWithSIDs.html

My favorite from there is "The day I write my last line of C code will be a very happy day for me!"